A common problem when providing electrical cables and equipment housings which are to be located underground is how to ensure that joints between cables and between cables and housings are perfectly sealed against the ingress of water and water vapour. The problem has become more severe since the use of cables with polyethylene sheaths has become widespread. These are more difficult to seal to each other and to housing entry glands than cables with lead sheaths for which conventional plumbing techniques were effective. The standard of integrity of the sealing of cables and housings required is very high because of the large number of seals which can exist in a system. Failure of any one seal can cause failure of the whole system.
Because of the difficulty of making perfect seals it is common practice to apply internal air pressure to both cables and housings. Leaks in them can then be detected by measuring the air flow in a dynamic system or the fall of pressure in a static system. These techniques are, however, expensive to implement, are not suitable for use with modern filled cables, do not give clear indications of the locations of leaks and are relatively insensitive requiring prolonged monitoring periods to detect small leaks.
Alternative methods of indicating the quality of seals of equipment housings are known in which the sealing members are duplicated in such a way as to enclose an air space between them. The air pressure in this space is raised relative to the atmosphere by an air pump and the pressure then monitored over a period by an air pressure gauge. Because the enclosed air space is smaller in volume than the whole housing the pressure falls more rapidly in the event of a leak than if the whole housing is pressurised and the time necessary to detect small leaks is reduced to a number of hours rather than days.